The present invention relates to a two-tube hydraulic shock absorber comprising coaxial outer and inner tubes or cylinders which between them define an interspace, wherein a piston is slidable in the inner cylinder and includes a rod which projects from an upper end of the shock absorber through an annular head common to both cylinders, the head having an annular radial layer shoulder which engages axially a rim at the upper end of the inner cylinder, wherein the head includes a bush in which the rod is slidable, and the head is formed with an annular reservoir overlying the bush for receiving hydraulic fluid which is drawn between the rod and the bush from the chamber in the inner cylinder, radial passages interconnecting the reservoir and the upper part of the interspace, and wherein the interspace contains gas under pressure, one-way sealing means being provided to prevent the gas under pressure from reaching the passages and at the same time to allow oil to descend into the interspace through the passages.
Throughout the present specification and claims terms such as `upper`, `lower`, and the like refer to the normal arrangement of a shock absorber installed in a motor vehicle suspension.
For some time two-tube shock absorbers have been known and used, in which the interspace between the two tubes or cylinders contains a gas under pressure. This gas under pressure acts on the reserve of hydraulic fluid in the lower part of the interspace and offers various advantages, among which is the reduced likelihood of the hydraulic fluid emulsifying or the fluid column being disrupted. By virtue of this pressurization, the hydraulic fluid can more readily reach the spaces which become available when the shock absorber is in use, and the latter can better fulfil its proper function.
Simple pressurization of a normal two-tube shock absorber is sufficient to improve its performance in the manner described above: the construction must be modified so that the gas exerts pressure only on the free surface of the hydraulic fluid in the lower part of the interspace and cannot reach the reservoir in the head, since in this case the gas under pressure passes through the interspace between the bush and the rod to act on the hydraulic fluid contained in the upper part of the inner cylinder above the piston. In this case, the levels of the hydraulic fluid in the inner cylinder and the interspace will tend, by the principle of communicating vessels, to equalize in an undesirable manner. On the other hand, the hydraulic fluid which is drawn to the reservoir between the bush and the rod must be able to flow back to the reserve in the interspace, such as occurs in non-pressurized two-tube shock absorbers. In order that this may happen without the transfer of gas under pressure to the reservoir in the head, one-way sealing means are associated with the latter, as known in the art.
In one embodiment, known from German Patent application No. 28 08 481 of Feb. 28th, 1978, the one-way sealing means comprise a skirt of rubber or a similar material which surrounds the head in the area where the radial downflow pipes emerge from the reservoir.
In another known embodiment, the reservoir in the head is absent and the hydraulic fluid drawn between the bush and the rod flows down radially over the head itself to feed longitudinal channels in the head, which in turn communicate with the interspace. At the entrance to the channels, the head incorporates a one-way annular valve with the above-described function.
Neither of the two known solutions take account of the fact that in two-tube shock absorbers the head is coupled to the inner cylinder by means of an annular boss inserted in the upper part of the inner cylinder, and by means of a radial shoulder which abuts the rim at the upper end of this same cylinder. This is not a very accurate coupling, since the shock absorber cylinders and the head are manufactured to rather wide tolerances. The coupling between the inner cylinder and the head does not afford tight sealing for the fluids and even less so for gas under pressure which may be drawn from the interspace directly into the upper space of the inner cylinder and give rise to the drawbacks which the above-mentioned one-way sealing means tend to eliminate.
If it is wished to use known one-way sealing means of the above type, an airtight seal has to be provided in the coupling between the inner cylinder and the head, for example, using an additional gasket, and this increases the cost.
The problem which forms the basis of the present invention is that of providing a two-tube hydraulic shock absorber which uses gas under pressure and does not have the above-mentioned disadvantage.